Palladium catalyzed carbon-carbon and carbon-heteroatom bond forming reactions, also referred to hereinafter as cross-coupling reactions, such as but not limited to conventional reactions such as Suzuki-Miyaura, Stille, Heck, Sonagashira, Negishi, Kumada cross-coupling, Buchwald-Hartwig aminations, catalytic ether formation, catalytic α-arylations of ketones, and catalytic thioether formation reactions, are extremely powerful synthetic tools in organic chemistry.
However, there are major limitations associated with the use of such reactions that would be advantageous to overcome. One such limitation is that traditional catalysts, such as palladium tetrakis(triphenylphosphine) or the catalysts formed in situ from an appropriate triarylphosphine and either a Pd(II) or Pd(0) precursor, have a generally low activity and therefore need to be present in comparatively high concentrations to realize high conversion rates. Both the high cost of palladium and the high costs associated with removing palladium metal residues due to spent catalyst in the product make the use of such high concentrations undesirable. Another limitation is that such traditional catalysts exhibit even lower activity in cross-coupling reactions that employ deactivated aryl bromides and are generally ineffective with respect to coupling aryl chlorides. As aryl chlorides are a particularly attractive class of substrate due to their greater availability and attractive costs, as compared to their bromide and iodide analogs, the ineffectivity of such traditional catalysts is problematic.
Research focusing on palladium compounds and their use in catalysis at both industrial and laboratory scales has increased over the past ten years. Ligandless systems are known and have been studied, however it is well understood that the ancillary ligation to the metal center plays a crucial role in dictating the efficiency of a catalytic system, thus such ligandless systems have not been particularly effective. As a result, bulky, electron-rich phosphines ligands such as P(t-Bu)2Me and P(t-Bu)3 have come to be commonly used to stabilize Pd(0) intermediates and hence have been seen to be effective. However, phosphine ligands have several drawbacks:                (1) they often are prone to air oxidation and therefore require air-free handling,        (2) when these ligands are subjected to higher temperatures, significant P—C bond degradation occurs, thus requiring the use of an excess of the phosphine, and        (3) they often react with Pd precursors, such as Pd(OAc)2, in a reduction process forming PnPd(0) and phosphine oxide;which limit their usefulness.        
As they represent an attractive alternative to tertiary phosphines in homogeneous catalysis, nucleophilic heterocyclic carbene (NHC) ligands have become increasingly popular in the last few years. In general, NHCs exhibit reaction behavior that is much different than phosphines, for example, displaying high thermal stability and tolerance to oxidation conditions. Several systems based on the combination of imidazolium salts (air-stable precursors to air sensitive NHC) and Pd(0) or Pd(II) sources have been developed to generate catalytically active species in situ, where such active species mediate numerous organic reactions, principally cross-coupling reactions. These preliminary systems and others have demonstrated the importance of the NHC/Pd ratio on the efficiency of the reactions, pointing to an optimum 1:1 ligand to metal ratio in most cases. From there, efforts have been aimed at the development of monomeric NHC-bearing Pd(II) complexes and the study of their catalytic activity. Generally, shorter reaction times are observed in these well-defined systems, since the carbene is already coordinated to the palladium center. Also, the use of a well-defined pre-catalyst allows for a better knowledge of the amount of ligand-stabilized palladium species in solution, by reducing the possibility of side reactions leading to ligand or palladium precursor decomposition prior to the coordination of the ligand.
The synthesis of monomeric (NHC)Pd(allyl)Cl complexes and (NHC)Pd(carboxylate) complexes have been reported among many architectures, and activation mechanisms and catalytic activities have been studied. The synthesis of most of these complexes is directly related to successful in situ systems involving the use of air sensitive NHCs and a corresponding palladium source. For example, a catalytic system for the Heck reaction involving the use of diazabutadiene ligands and Pd(OAc)2 or Pd(acac)2 as palladium precursors has been reported.
2,4-Pentadione (acetylacetone, Hacac) and other β-carbonyl compounds are very versatile and are common ligands in transition metal chemistry since they are generated on an industrial scale. 2,4-Pentadione typically binds metal ions in a η2—O,O fashion, although some other coordination modes have been observed in platinum (II) and palladium(II) complexes. Previous work has focused on the reactivity of palladium(II) acetylacetonate and related compounds with phosphines leading to new complexes, but no catalytic applications were reported. Recently, others have extensively researched the use of such types of complexes as hydrogenation catalysts.
Thus, the use of currently known catalyst systems for carbon-carbon and carbon-heteroatom formation can often be problematic. Such issues include (i) relatively difficult or laborious synthesis, (ii) expensive catalyst precursors and/or ligand sets, (iii) oxygen and water sensitivity of ligands or metal catalysts, (iv) optimization of activity is difficult because of disparate pre-catalyst structures and unusual and non-modular ligands, and (v) catalysts for the activation of aryl chloride need to be used in relative high concentrations. Therefore, it would be desirable to have catalysts or catalyst systems that provide solutions to the above-identified problems. It would also be desirable if such catalyst systems could be used directly on the industrial scale without the need for their isolation and are stable over wide ranges of temperature and pressure. Further, it would be desirable if such complexes could be prepared in a single reaction step from readily available starting materials.